This was the second year for this project which is using a combination of methods to analyze the ion transport properties of lysosomal membranes. Lysosomes are intracellular organelles that serve in most cells as digestive organelles although in some tissues they are used for other functions. Disorders of lysosome function lead to a variety of diseases including neurological dysfunction (lysosomal storage diseases) and osteopetrosis (overcalcification of bone). Lysosomes utilize an ATP-driven proton pump to maintain an acidic luminal pH and facilitate their digestive function. Such a pump can only be effective if accompanied by additional ion transport to dissipate the transmembrane voltage built up by the ATPase. Genetic evidence suggests that this additional ion pathway a ClC-type anion transporter, but functional experiments have not yet demonstrated such a pathway. We seek to identify the ion transport pathways in lysosomes, to characterize their properties, and to identify the responsible proteins. This year we characterized in detail the Cl permeability that we discovered last year. We demonstrated that this permeability stems from a Cl-/H+ antirporter rather than an ion channel and revealed its stoichiometry as 2Cl-/1H+. We further showed that this antiporter is the major Cl- permeability of lysosomes. Using siRNA knockdown experiments we further demonstrated that ClC-7, a member of a family of anion transport proteins, is responsible for the lysosomal Cl- conductance. This results have implications for organellar pH regulation as well as for human disease, as blockers of ClC-7 might act as therapies for osteoporosis.